1. Field of the Invention
The present invention relates to a nitride-based semiconductor laser device.
2. Description of the Related Art
A blue-violet semiconductor laser device of 400 nm band range is promised for next-generation DVD (digital versatile disk) and the like. The blue-violet semiconductor laser is configured as a ridge-shaped waveguided semiconductor laser device.
Such a blue-violet semiconductor laser device as described above is disclosed in “Reference 1” wherein the active layer is configured as a multilayered quantum well structure. In this case, there are some disadvantages of the increase of the operation voltage, the increase of the Stark effect due to the piezoelectric field and the increase of the threshold voltage.
In this point of view, it is desired that the number of the layers constituting the multilayered quantum well structure is decreased and the thickness of each well layer composing the same multilayered quantum well structure is decreased. In this case, however, the total thickness of the multilayered quantum well structure is decreased so as to cause the overflow of carriers and thus, deteriorate the luminous efficiency. As of now, therefore, in the semiconductor laser device configured such that the active layer is formed as the multilayered quantum well structure, the reduction of the operation voltage and the reduction of the threshold voltage can not be established under the prevention of the overflow of the carriers yet.
In the ridge-shaped waveguided semiconductor laser device as disclosed in Reference 1, in contrast, the multilayered quantum well structure is formed at a lower temperature due to the material compositions thereof and the cladding layer(s) and the optical guide layer are formed at a higher temperature due to the material compositions thereof. Namely, although the multilayered quantum well structure is adjacent to the optical guide layer and/or the cladding layer(s), they can not be formed subsequently in the practical manufacturing process due to the difference in formation temperature therebetween. In this point of view, the multilayered quantum well structure, the optical guide layer and the cladding layer(s) are formed intermittently due to the control in the formation temperature thereof.
As a result, some defects are likely to be formed in the well layers of the multilayered quantum well structure. Moreover, the piezoelectric effect may be generated due to the distortion from the crystal interface so as to cause the distortion of the band structure of the multilayered quantum well structure and thus, disturb the recombination of the carriers, leading to the trouble in luminescence. In addition, the voltage drop occurs at the crystal interface to increase the operation voltage. Moreover, some defects may be enlarged due to the heat generation from the long-term use, leading to the deterioration of the reliability of the multilayered quantum well structure, that is, the ridges-shaped semiconductor laser device.
[Reference 1] JP-A 10-256657 (KOKAI)